Process for coating ferrous surfaces



United States Patent 3,547,711 PROCESS FUR COATING FERROUS SURFACES Ronald A. Ashdown, London, England, assignor to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed Aug. 15, 1968, Ser. No. 752,806 Claims priority, application Great Britain, Aug. 18, 1967, 38,130/67 Int. Cl. C23f 7/10 US. Cl. 148-615 10 Claims ABSTRACT OF THE DISCLOSURE A process for forming a phosphate coating on steel surfaces which are normally difiicult to phosphate, such steel surfaces being characterized as being high alloy steels or low alloy steels which include one or more elements that promote the formation of a highly tenacious oxide surface film. The steel surface is first treated with an oxalate solution to form an oxalate coating on the surface and the thus-coated surface is then given a conventional phosphate treatment. The treatment with the oxalate solution is carried out to the extent that an oxalate coating is formed on the surface, prior to treatment with the phosphate coating solution.

This invention relates to the phosphate coating of ferrous surfaces and, more particurally, relates to the formation of phosphate coatings on steel surfaces which cannot be coated, or cannot be coated satisfactorily, by conventional processes employing a single phosphate coating stage.

The use of phosphate coatings on ferrous surfaces is well known in the art. Such coatings provide corrosion protection and/or a base for paint or other siccative coatings or as a substrate for the application of a cold-forming lubricant. Additionally, where the phosphate coating is a manganese phosphate coating, the coating is useful as a lubricant or for retaining a lubricant for hearing or sliding surfaces.

It has been found, however, that with some types of ferrous surfaces, a satisfactory phosphate coating cannot be obtained by conventional one-stage phosphate coating techniques. The types of steel surfaces with which coating difiiculties are encountered fall within two classes, high alloy steel, i.e., steels which incorporate a high proportion of alloying elements, and low alloy steels which include one or more elements that promote the formation of a highly tenacious oxide surface film. Attempts to form a phosphate coating by conventional phosphating techniques on such surfaces have, generally, not been successful, the coatings where produced, being uneven or of poor quality.

It is, therefore, an object of the present invention to provide an improved process for the phosphate coating of steel surfaces which surfaces have not heretofore been successfully coated by conventional phosphating techniques.

A further object of the present invention is to provide an improved process for coating difiicult to phosphate steel surfaces, which process consistently produces a high quality phosphate coating.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes a process for treating difficult to phosphate steel surfaces which comprises treating the steel surface with an oxalate coating solution, forming an oxalate coating on the steel surface thus-treated, treating the oxalate-coated 3,547,711 Patented Dec. 15, 1970 surface with a phosphating solution and forming a phosphate coating on the thus-treated surface. By means of this process, a finely-crystalline phosphate coating is formed on the steel surfaces treated.

More specifically, in the practice of the method of the present invention, it has been found that the high quality, finely-crystalline phosphate coatings are produced on any steel surface treated in accordance with the present method. The most striking improvements, however, are obtained when this method is used to treat so-ca1led, difficult to phosphate steel surfaces, which surfaces have not been satisfactorily phosphate coated by the previous conventional processes.

By difficult to phosphate steel surfaces, it is intended to refer to the high alloy steels which contain a high proportion of alloying elements and also low alloy steels which include one or more elements which promote the formation of a highly tenacious oxide surface film on the steel. Exemplary of the high alloy steels of this type are the recently developed steels which are aged-hardenable in the martensitic state, which steels are commonly referred to as maraging steels. Typical of such steels are those containing about 18% nickel with cobalt, molybdenum, titanium; those containing about 12% nickel, 5% chromium and 5% molybdenum; as well as stainless steels which contain high proportions of nickel and/or chromium, such as 18/ 8 stainless steel. Exemplary of the low alloy steels of this type which are difficult to phosphate satisfactorily is a steel which contains about 1% chromium with from about 0.2 to 0.4% molybdenum. In such steel, it is believed that the molybdenum increases the tenacity of the chromium oxide film on the steel surface. It is believed that other steels of the above type, which have heretofore been difiicult to phosphate, and with which the method of the present invention is advantageously used, will be apparent to those in the art from the examples of typical steels of this type which have been given.

The steel surfaces to be treated are preferably pickled or grit-blasted before being subjected to the coating process. A suitable pickling solution which may be used is one containing about 25% by volume of concentrated nitric acid and 10% by volume of 60% hydrofluoric acid. As will be apparent to those in the art, other suitable pickling acids may also be used. Typically, the steel surface is treated with the pickling acid for about 1 minute, at ambient temperature, and may, thereafter, be rinsed with cold water. Alternatively, the steel surface to be treated may be grit-blasted. Typically, this may be carried out using grade 12 cast iron grit, 60/80 mesh alumina, or /200 mesh alumina.

It has been found that the final phosphate coating obtained by the process of the present invention Will vary depending upon the pickling or grit-blasting treatment which is used. In this regard, it has been noted that generally, the heavier coating weights are obtained where the surface of the steel has been blasted With grade 12 cast iron grit, while a finer more even coating is generally obtained after pickling or blasting with 60/80 mesh aluminum. It is to be appreciated, however, that suitable phosphate coatings are produced by the present method on steel surfaces which have not been subjected to either the pickling or grit-blasting operation.

The steel surfaces to be treated, either as such, or after a pickling or grit-blasting treatment, are contacted with an oxalate coating solution. This contact of the steel surface and the coating solution may be carried out in any convenient manner, as for example, by immersion, spraying, flowing, flooding, or the like techniques. The oxalatecoating solutions used may be any of the conventional oxalate-coating forming solutions as are known to those in the art. Typically, these oxalate coating forming solutions are aqueous acidic solutions containing oxalic acid, or an oxalate salt, such as ferric oxalate as well as one or more accelerating compounds, such as thiosulfates, hydrosulfites, tetra-thionates, sulfites, ferricyanides, thiocyanates, nitrobenzene sulfonates, which compounds are typically added as the alkali metal or ammonium salts, alkali metal or ammonium halides, and the like. Typically, these solutions contain the oxalic acid or oxalate compound in amounts within the range of about 2 to 10% by weight, while the accelerator compounds are typically present in amounts up to about 5% by weight.

The treatment of the steel surfaces with the oxalate coating forming solution is carried out so as to effect the formation of an oxalate coating on the steel surface. In this regard, it is to be noted that a mere pretreatment in an oxalic acid solution is not effective in the present method and it is necessary that an oxalate-coating-forming solution be used and that the steel surfaces be treated with this solution for a period sufficient to form an oxalate coating on the surface. Typically, the treating time with such oxalate coating solutions will be from about 30 seconds to 30 minutes, although both shorter and longer treating times may be used in some instances provided the treating time is effective in forming an oxalate coating on the treated surface. Preferably, the oxalate coating solutions are, at elevated temperatures, temperatures from about 65 to 95 degrees C. being typical, although lower temperatures are also suitable with longer treating times.

Following the treatment of the steel surfaces with the oxalate coating solutions and the formation of the desired oxalate coating on the surface, the surfaces may, if desired, be water rinsed. Although this water rinse, which may be either hot and/ or cold water, is not essential in the present process, it is frequently desirable in order to prevent the carry over of any oxalate coating solution into the subsequently applied phosphating solution.

The oxalate coated steel surface is then treated with a phosphating solution. The treatment of the oxalate coated surfaces with the phosphating solution may be carried out in any convenient manner, such as by using immersion, spraying, flooding, flowing, or the like techniques. Any of the various conventional phosphate coating solutions as are known in the art, may be used, including zinc, iron and manganese phosphate coating solutions. Typically, these solutions are aqueous acidic solutions containing zinc, and/or manganese, and/or iron, phosphate ions, as well as various accelerators, such as nitrite ions, nickel ions, chlorate ions, and the like. The choice of the particular phosphating solution will, of course, depend upon the particular end use of the coated steel surface. Thus, as is known in the art, the zinc phosphate coatings are of particular value in providing corrosion resistance, a base for paint, as well as a lubricant base for surfaces which are to be cold formed; while the manganese phosphate coatings are useful as lubricant coatings or for retaining lubricants, such as for the run ning in of gears, cams, or other bearing or sliding surfaces.

The treatment of the oxalate coated surfaces with the phosphate coating solution is carried out for a period sufficient to form the desired phosphate coating on the surface. Typically, the treating times may be from about 1 minute to about 1 hour, although other treating times may also be suitable, provided the desired phosphate coating is formed on the surface. As with the oxalate coating solution, the phosphating solution is preferably used at an elevated temperature, temperatures of from about 60 to 90 degress C. being typical, although lower temperatures may also be used with longer treating times. Following the treatment with the phosphating solution and the formation of the desired phosphate coating, the treated surfaces may, if desired, be again rinsed with water and then dried. By treatment in accordance with this method, it is found that a uniform, finely crystalline phosphate coating is formed on the steel surfaces treated, even where the steel surface is a difiicult to phosphate steel.

In order that those skilled in the art may better understant the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, temperatures are in degrees centigrade and parts and percents are by weight. It is to be appreciated, however, that these examples are merely exemplary of the present invention and are not to be taken as a limitation thereof.

EXAMPLE 1 Clean panels of maraging steel containing 12% by weight nickel, 5% chromium and 5% molybdenum *were immersed for one minute in an oxalate-coating-forming solution at about 93 degrees centigrade, which solution contained the following components in the amounts indicated:

Components: Grams per liter Oxalic acid (expressed as C 0 40 Sodium metal dinitrobenzene sulfonate (expressed as NO 1.5 Ammonium thiocyanate (expressed as SCN) 2.6 Ammonium bifluoride (expressed as F) 5.0

After one minute, the panels were withdrawn, rinsed with water and then immersed for 30 minutes in a phosphating solution, at about 82 degrees centigrade, which solution contained the following components in the amounts indicated:

Components: Grams per liter Po 18.0 N0 25.0 Zinc Iron 2.0

After withdrawal from the phosphating solution, the panels were again rinsed with water and dried in an oven. A dark gray uniform, finely crystalline phosphate coating was found to be formed on the surface of each panel treated.

EXAMPLE 2 Panels of maraging steel containing 12% nickel, 5% chromium and 5% molybdenum were immersed for varying lengths of time in the oxalate-coating-forming solution of Example 1. The panels were then rinsed with cold water, rinsed with hot water and then immersed for 30 minutes in the phosphating solution of Example 1. The panels were then rinsed again with cold water, rinsed with hot water and dried in an oven for 10 minutes at degrees centigrade. In each instance, the coatings produced were uniformly crystalline and gray-green in color, the color being more pronouncedly green on the panels which have been subjected to the oxalate coating solution for the longer periods of time. The weight of the phosphate coating on the panels thus-treated was then determined using the caustic soda method. In this method, the coated panels were weighed, the coating was stripped from the panel by dipping it into a caustic soda solution, the panel was then rubbed with steel wool and then dried and reweighed. Using this procedure, the following results were obtained:

Time of immersion in oxalate- Weight of phosphate coat coating-forming solution ing formed in milligrams (min.) per square foot 1 110 While there have been described various embodiments of the invention, the compositions and methods described are not intended to be understood as limiting the scope of the invention as changes therewithin are possible and it is intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A process for treating a steel surface which comprises treating the surface with an aqueous acidic oxalate coating solution, forming an oxalate coating on the surface thustreated, treating the oxalate coated surface with an aqueous acidic phosphating solution and forming a phosphate coating on the thus-treated surface.

2. The process as claimed in claim 1 wherein the phosphate coating formed on the treated surface is a zinc phosphate coating.

3. The method as claimed in claim 2 wherein the steel surface treated is a high alloy steel.

4. The process as claimed in claim 2 wherein the steel surface treated is a low alloy steel which contains at least one element that promotes the formation of a highly tenacious oxide surface film on the steel.

5. The process as claimed in claim 3 wherein the steel surface is pickled prior to the application of the oxalate coating.

6. The process as claimed in claim 3 wherein the steel surface is grit-blasted prior to the application of the oxalate coating.

7. The process as claimed in claim 4 wherein the steel surface is pickled prior to the application of the oxalate coating.

8. The process as claimed in claim 4 wherein the steel surface is grit blasted prior to the application of the oxalate coating.

9. A coated steel surface prepared in accordance with the process of claim 3.

10. A coated steel surface prepared in accordance with the process of claim 4.

References Cited UNITED STATES PATENTS 2,164,042 6/1939 Rornig l486.l4

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 1486. 14, 31.5 

